Apatite (U-Th)/He (AHe) thermochronology depends on accurate knowledge of how diffusion occurs. This involves measurement of core diffusion kinetics as well as understanding the behavior of migrating He atoms. Drawing from previous studies as well as data obtained via continuous ramped heating (CRH), we assess several processes that need to be integrated into a single model for He diffusion in apatite. CRH analyses conducted at different heating rates show a kinetic response for both the “normal” lower-temperature and the higher-temperature release peaks, with peaks shifting to lower temperatures at lower heating rates. Where we do see a rollover in Arrhenius trends it also shows a kinetic response, being deferred to higher temperatures at higher heating rates, though many samples with unimodal release peaks do not show a significant rollover; fluorapatites seem to show more prominent rollover. For samples showing multiple release peaks, we find that their Arrhenius data often transition from one lower-temperature trend to another at higher temperatures that has about the same slope and thus activation energy. This looks very much like MDD behavior in K-feldspar, and MDD domain analysis fits the observed data very well, even if mechanisms involving discrete domain sizes are implausible. This interesting and unexplained result must speak to the nature of what is happening during analysis of samples having trapped He. To explore our data, we coded a simple diffusion model in which single He atoms are free to jump within a grid, but can also arrive at grid nodes designated as reversible sinks, escape from which depends on an exponentially temperature- dependent probability. The model includes radiogenic He production over geological thermal histories followed by laboratory CRH outgassing. When conditioned using D values observed for AHe, the model accurately predicts parameters such as closure temperature and fractional loss. When traps are introduced, the model simulates the essential nature of the dual-peak CRH results we see. Three important results emerge from this model. (1) Few sinks need be present. (2) Trapping occurs twice during diffusion, first in nature and then again during laboratory outgassing, meaning that the ratio of the gas amounts beneath each CRH peak overestimates the geological trapping. (2) Trapping in nature is very dependent on the sample’s thermal history: it is smallest for ancient rapid cooling and largest for samples that reside in the PRZ (allowing more radiogenic production to find traps before diffusion ceases). This model raises the possibility that complex CRH data record extended thermal-history information. If CRH and 4He/3He analysis were combined the 3He lab outgassing would record the sample’s trapping dynamics, and the 4He outgassing would reflect that plus a segment of the sample’s thermal history, which could be extracted using the 3He observations.

Apatite – Ca5(PO4)3(OH,F,Cl) – is a common accessory mineral in many terrestrial rocks and has a low (~75˚C) closure temperature for He retention[1]. As such the (U-Th-Sm)/He thermochronometer has become one of the most used dating techniques to constrain low temperature cooling histories of terrestrial samples. Apatite has been documented in many meteoritic, lunar, and martian samples, and is a particularly common accessory phase in many martian lithologies[2]. In meteoritic samples, (U-Th-Sm)/He dating of apatites provides some methodological challenges in that grains are frequently anhedral making it complicated to do a FT correction[2]. Results are commonly interpreted to represent the timing of shock metamorphism related to impacts onto the (planetary) surface which often correlates with cosmic-ray exposure ages[2]. NWA 7034, and its pairings, represents a piece of martian regolith that documents events on the martian surface that span approximately 4 Ga[3]. NWA 7034 is known to contain upwards of 4 wt.% apatite as detrital mineral fragments and as accessory phases in many lithic and impact-derived clasts[4]. These apatites have been previously analyzed geochronologically via U-Pb apatite and isotopically via δD, δ37Cl, and H2O using microbeam techniques[5,6]. At present, (U-Th)/He dating of NWA 7034 and its pairings has been restricted to bulk rock analyses[4,7,8,9]. This whole-rock approach relies on assumptions that the He, and its parent isotopes, are evenly distributed throughout the sample and that resetting of the WR-He thermochronometer is universal throughout the sample in response to the thermal pulse associated with the impact. These studies have yielded widely dispersed datasets that range from 50 Ma to 200 Ma, putting some of those assumptions into question[4,7,8,9]. In this contribution, we utilize the laser ablation double-dating (LADD) technique on individual apatites present within several polished slabs of NWA 7034. This targeted approach allows us to acquire (U-Th-Sm)/He ages, U-Pb ages, and trace element information within a petrological framework. The (U-Th-Sm)/He ages will provide additional constraints on the low temperature history of NWA7034 and help test the veracity of WR-He retention ages. References: [1] Zeitler et al. 1987 GCA, [2] Min 2005 Rev. Mineral Geochem; [3] Cassata et al. 2018, SciAdv; [4] Agee et al. 2013 Sci; [5] Hu et al. 2019 MAPS; [6] Davidson et al. 2020 EPSL; [7] Cartwright et al. 2014 EPSL; [8] Lindsay et al. 2021 MAPS; [9] Stephenson et al. 2017 MAPS

Thermal history models are interpretive tools that incorporate data from chronometers and implement the published kinetics in the context of independent constraints on a sample’s known geologic history, in order to explore specific gaps in geologic knowledge. Despite their central role in the interpretation of thermochronologic datasets, our community has no standards for what characterizes a “robust” thermal history model result, how a model result’s rigor can and should be demonstrated, and how to communicate the key layers of interpretation produce a preferred thermal (and geologic) history. As a result, and through no fault of any one study or modeling program, published models are a patchwork of modeling philosophies, assumptions, and auxiliary hypotheses that are rarely sufficiently explored—to the frustration of authors, reviewers, and readers. This patchwork can give rise to conflicting conclusions and generate apparent controversies that distract from the geologic questions at hand. Therefore, our community needs to both embrace a diversity of modeling approaches and collectively discuss and set broad expectations for what constitutes thermal history modeling best practices. Here, we argue that the fundamental characteristic of any robust thermal history model result is that it is accompanied by a clear articulation of the “why”—e.g., the reason(s) that a model produces a distinctive history, be it the power of a geologic constraint, a grain’s age, a spatial relationship between samples, the choice of kinetic model, etc. We demonstrate this approach using (U-Th)/He data from basement rocks in the Front Range, CO, which when modeled require a distinctive Neoproterozoic thermal history: heating to 235-280°C after ca. 650 Ma and then cooling to <60°C during Paleozoic time. We demonstrate “why” through a suite of models that add, modify, and remove geologic constraints and data from the preferred model. We find that a heating event is required to produce the observed zircon He age-[eU] trend because (1) there is no more than ~600 My of radiation damage accumulated in the zircon crystals, (2) the geologic record places the samples at the surface prior to 650 Ma, and (3) published Ar ages require that these rocks were colder than ~250˚C for most of the last 1.5 Gy. By identifying these key factors, our sensitivity test facilitates comparisons to other studies and directs further discussion to how confident we are in the parts of our data and model set-up that produce this distinctive result. More broadly, this exercise demonstrates one of the challenges of deep-time thermochronology: the potential to accumulate multiple auxiliary assumptions that control the model result in ways that are not obvious, even to the experienced model user, without deliberate exploration of alternative solutions—further underscoring the need for more open discussion of this topic.

Most thermochronological studies aimed at constraining exhumation rates rely on bedrock datasets. Often, they involve the analysis of samples collected along an elevation profile in terrains with high relief. However, there are several limitations to this approach, most importantly access to an appropriately steep traverse and sufficient relief to overcome uncertainties, and to have a broad enough range in closure ages as a function of elevation. Detrital thermochronology offers an alternative approach which can mitigate these challenges through coordinated dating of modern river sediments using multiple thermochronologic methods. Modern detrital sediments from active catchments provide an excellent source of material, typically rich in rock-forming and accessory minerals. Detrital thermochronologic data for material in sedimentary basins has been used widely to infer exhumation histories of sedimentary source terrains, reconstruct paleorelief, and evaluate spatial and temporal variations in erosion rates; however, there have been comparably fewer studies that apply this technique to evaluate regional exhumation patterns using detrital samples from active catchments. Based on the approach presented in Gallagher and Parra, (2020), we are exploring the capability of detrital thermochronologic data to infer regional exhumation patterns in the southeastern Sierra Nevada, CA. Here the uplift history remains debated and the potential mechanism of uplift has yet to be thoroughly constrained. Many catchments along the eastern side of the Sierra Nevada exhibit advantageous characteristics for detrital thermochronologic studies, including steep topography and high relief (that make it more difficult to sample bedrock), limited lithologic variability (which minimizes point-source biasing), relatively simple geologic structure, and relatively easy access to detrital sampling localities. Additionally, the dominant source of the southeastern Sierra Nevada catchments, the igneous units of the Sierra Nevada batholith, include abundant rock-forming minerals for 40Ar/39Ar thermochronology (hornblende, biotite, and sometimes muscovite) as well as abundant accessory minerals for (U-Th)/Pb geochronology(zircon), (U-Th)/Pb thermochronology (apatite), and (U-Th)/He thermochronology (zircon and apatite). Collectively, detrital thermochronological data from these minerals can elucidate much of the post-crystallization thermal history of the eastern flank of the Sierra Nevada. Preliminary results of this technique demonstrate the potential of this cost- and labor-efficient approach for exhumation history studies.

The Ultraviolet Laser Ablation Microprobe (UVLAMP) method of releasing helium from samples is an excellent, but under-utilized, tool in the diverse toolkit of gas extraction approaches available to researchers working with the (U-Th-Sm)/He thermochronology method. So far, most applications have involved some form of Laser Ablation (U-Th-Sm)/He dating (LAHe) or combined LAHe and Laser Ablation U-Th/Pb double dating (LADD) (e.g. 1, 2, 3, 4, 5, 6, 7). Other applications using UVLAMP have focused on 2D-mapping of helium distributions within zircon crystals (8) and stepwise Laser Ablation Depth Profiling (LADP) of induced helium diffusional loss profiles in apatite and zircon (9, 10). Based on the latter examples the stepwise helium LADP method would appear to be an excellent method to study the intricacies associated with a variety of aspects of the (U-Th-Sm)/He dating method and the interpretation and modeling of its results. Given that it creates high resolution helium profiles from the crystal margin to its core without the need to heat the sample to release the gas. Thus, it avoids issues of within-experiment radiation damage annealing, diffusional flattening of helium zonation, and/or the sudden release of helium from fluid and/or melt inclusions that can be associated with approaches using step heating of samples to acquire similar information about the helium distribution within a sample. In this contribution we focus on the results of high spatial resolution helium LADP experiments in a variety of accessory minerals (apatite, zircon, monazite, and titanite). The experiments are intended to a) empirically determine the alpha ejection distance and how those results compare to the distance for each mineral derived from SRIM calculations (11) and b) image natural helium distribution profiles from rim to core in zircons to produce data that are equivalent to those produced by 4He/3He thermochronology (12) experiments, but without the need to proton irradiate the sample. Initial LADP results on Durango apatite yielded an alpha ejection distance that is within error of the theoretical value, while results from several larger (>5 mm) zircon crystals did not yield profiles consistent with the presence of a straightforward alpha ejection zone. The helium depth profile results from the zircons were suggestive of either natural diffusional loss profiles, showing evidence of U-Th zoning, or a combination thereof. 1 Boyce et al. GCA 70, 2006; 2 Vermeesch et al. GCA 79, 2012; 3 Tripathy-Lang et al. JGR-ES 118, 2013; 4 Evans et al. JAAS 30, 2015; 5 Horne et al. GCA 178, 2016; 6 Horne et al. CG 506, 2019; 7 Pickering et al. CG 548, 2020; 8 Danisik et al. Sci Adv 3, 2017; 9 Van Soest et al. GCA 75, 2011; 10 Anderson et al. GCA 274, 2020; 11 Ziegler and Biersack, 1985; 12 Shuster and Farley EPSL 217, 2004.

Zircon Raman dating is an emergent thermochronological method. It exploits the disruption of the zircon lattice due to α-disintegration of trace amounts of 238U, 235U, 232Th, and their daughters. The radiation damage broadens the Raman bands and shifts them to lower wavenumbers. The Raman bandwidths provide a sensitive measure for the accumulated lattice damage (Nasdala et al., 1995; Nasdala et al., 2001). The measured bandwidth and the effective uranium concentration define the Raman age (Härtel et al., 2021). Radiation damage anneals upon heating and the meaning of the Raman age depends on the zircon’s thermal history. The Raman age is a formation age if no annealing has taken place, or a reset age if all pre-existing damage has been annealed by a geological heating event. In the case of partial annealing, however, the Raman age is a mixed age with no obvious geological significance. Mixed ages are difficult to interpret and cannot be distinguished from reset ages using the standard procedure for annealing detection (Nasdala et al., 2001). On the other hand, inhomogeneous damage distributions due to actinide zoning within zircon grains present a problem for zircon Raman dating. Overlapping signals from more and less damaged zones lead to asymmetric Raman bands and overestimated bandwidths (Nasdala et al., 2005). We discuss Raman spectra of zircon from partially annealed samples and spectra with asymmetric bands. We introduce discrimination plots based on the 356, 439, and 1008 cm-1 bandwidths that provide a means for detecting and distinguishing asymmetry and partial annealing. We discuss examples of zircon Raman dating and present a measurement protocol.

A compilation of legacy and new low-temperature thermochronological data from the British-Irish Isles and their surrounding offshore shelves yielded c. 700 AFT ages and c. 180 AHe ages from 29 peer-reviewed papers, 27 Geotrack industry reports and several new unpublished studies from offshore Ireland. The compilation shows for the first time a regional age pattern, with older AFT ages in Scotland and Northern Ireland than in the rest of Ireland. This pattern is tentatively attributed to the influence of the Anton-Dohrn Transfer Zone (ADTZ) during an Early Cretaceous phase of plate-wide uplift that resulted in more exhumation to the SW of the transfer zone than to the NE. Caledonian faults might also create differential exhumation of the tectonic blocks between them, as is observed in the compilation of AFT data from northern Scotland and this could explain the dispersion in the timing of exhumation seen on the North Porcupine High, offshore Ireland. Finally, the Paleogene exhumation visible in the Central Irish Sea, and attributed in recent years to igneous underplating, has not been detected in the Malin Sea-Outer Hebrides, despite the area being underlain by a high-velocity body also interpreted as igneous underplating. In conclusion, a detailed analysis of a large dataset of low-temperature thermochronological has revealed the possible influence of major crustal structure on the Mesozoic exhumation of this part of the NE Atlantic Margin, with large-scale decoupling occurring at a transfer zone and medium-scale decoupling occurring along regional-scale faults. The dataset also shed some doubts on the generic nature of exhumation caused by igneous underplating which has been much discussed in recent years.

We performed continuous ramped heating (CRH) on apatites from various tectonic settings and found two major types of 4He outgassing behavior. Apatites with good (U–Th)/He age reproducibility show simple and unimodal incremental gas-release curves that are similar to those predicted by volume diffusion, whereas samples exhibiting greater age dispersion have complex gas-release curves that feature He ‘spikes’ and secondary gas-release peaks deferred to higher temperatures. Age dispersion from the apatites with simple outgassing behavior can be explained by variability in their relative He retentivity observed on Arrhenius arrays—with similar activation energy but different diffusivities, which may be a result of fine-scale crystal imperfections. The observed high-temperature gas component and the resulting “too-old” AHe ages, combined with an attempt at age correction based on secondary peak gas-removal, seem to indicate the existence of sink-like crystal imperfections that can trap 4He both in nature and during laboratory heating. CRH analysis at different heating rates further suggests that the second gas-release peak occurs at varying temperatures, indicating that the sink is kinetically responsive, and if characterizable, may contain additional thermal-history information. These observations suggest that (1) CRH can be deployed as a routine screening tool for (U–Th)/He dating, (2) diffusion of 4He could be complicated by imperfections beyond radiation damage, and (3) if the proposed sinks exist and retain appreciable 4He, there are opportunities to explore additional thermal histories of natural samples.

Abstract A new method for automatic counting of etched fission tracks in minerals is developed and recently published in Geochronology (see Nachtergaele and De Grave, 2021). Artificial intelligence techniques such as deep neural networks and computer vision were trained to detect fission surface semi-tracks on images. The deep neural networks can be used in an open source computer program for semi-automated fission track dating called “AI-Track-tive”. Our custom-trained deep neural networks use YOLOv3 object detection algorithm, which is currently one of the most powerful and fastest object recognition algorithms. Two Deep Neural Networks were trained for both apatite and mica using our training dataset with images from the available microscope. The developed program successfully finds most of the fission tracks in the microscope images, however, the user still needs to supervise the automatic counting. The presented deep neural networks have high precision for apatite (97%) and mica (98%). Recall values are lower for apatite (86%) than for mica (91%). These high values have been obtained on images using the same microscope that provided the training images. The application can be used online on the web page https://ai-track-tive.ugent.be or after download as an offline application for Windows. The online application can be used to analyse captured images and does not require installation or download. The offline application can be used for both live track recognition on live microscopy images and captured images of apatite or mica. AI-Track-tive is written in Python and can be downloaded on https://github.com/SimonNachtergaele.

Arc-arc collision plays an important role in the formation and evolution of continents (e.g., Yamamoto et al., 2009; Tamura et al., 2010). The Izu collision zone central Japan, an active collision zone between the Honshu Arc and the Izu-Bonin Arc since the middle Miocene (Matsuda, 1978; Amano, 1991; Kano, 2002; Hirata et al., 2010), provides an excellent setting for reconstructing the earliest stages of continent formation. Multi-system geo-thermochronometry was applied to different domains of the Izu collision zone, together with some previously published data, in order to reveal mountain formation processes, i.e., vertical crustal movements. For this study nine granitic samples yielded zircon U–Pb ages of 10.2–5.8 Ma (n = 2), apatite (U–Th)/He ages of 42.8–2.6 Ma (n = 7), and apatite fission-track (AFT) ages of 44.1–3.0 Ma (n = 9). Thermal history inversion modelling based on the AFT data using HeFTy ver. 1.9.3 (Ketcham, 2005), suggests rapid cooling events confined to the study region at ~5 Ma and ~1 Ma. The Kanto Mountains are thought to be uplifted domally in association with collision of the Tanzawa Block at ~5 Ma. But this uplift may have slowed down following migration of the plate boundary and late Pliocene termination of the Tanzawa collision. The Minobu Mountains and possibly adjacent mountains may have been uplifted by collision of the Izu Block at ~1 Ma. Mountain formation in the Izu collision zone was mainly controlled by collisions of the Tanzawa and Izu Blocks and motional change of the Philippine Sea plate at ~3 Ma (Takahashi, 2006). Earlier collisions of the Kushigatayama Block at ~13 Ma and Misaka Block at ~10 Ma appear to have had little effect on mountain formation. Together with ~90° clockwise rotation of the Kanto Mountains at 12-6 Ma (Takahashi & Saito, 1997), these observations suggest that horizontal deformation predominated during the earlier stage of arc-arc collision, whereas vertical movements due to buoyancy resulting from crustal shortening and thickening developed at a later stage. References: Amano, K., 1991, Modern Geol., 15, 315-329; Hirata, D. et al., 2010, J. Geogr., 119, 1125-1160; Kano, K., 2002, Bull. EQ Res. Inst. Univ. Tokyo, 77, 231-248; Ketcham, R.A., 2005, Rev. Min. Geochem., 58, 275-314; Matsuda, T., 1978, J. Phys. Earth, 56, S409-S421; Takahashi, M., 2006, J. Geogr., 115, 116-123; Takahashi, M. & Saito, K., 1997, Isl. Arc, 6, 168-182; Tamura et al., 2010, J. Petrol., 51, 823, doi:10.1093/petrology/egq002; Yamamoto, S. et al., 2009, Gond. Res., 15, 443-453.